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Fléchette ammo for modern sniper rifles?

Evidently those old systems did not result in sufficient increase in lethality for the standard rifleman to justify the cost of purchase. This makes me wonder, though, if the system might find more utility today in the hands of military snipers. With weapon systems like the 338 Lapua pushing out to 2000 yards, would a faster fléchette round with a flatter trajectory allow snipers to increase their lethality at very long range? Would it be worthwhile for militaries to invest in these systems for their snipers?

DARPA actually has a functional .50 cal guided bullet system undergoing development. They put out a demonstration video that shows both the predicted ballistic path of a bullet and the actual shot as it tracks towards a moving target. They have an expert shooter take the first several shots, then allow an untrained novice to try it out.

Flechette rounds might have high muzzle velocity, they lose that quickly due to low mass compared to traditional full sized rounds. Lighter rounds are also more subject to effects of cross wind. Lower mass is easier to influence than larger mass. Predictability is also important in accuracy. I know for a fact that heavy and light bullets all drop at the same rate. A high speed round with flatter trajectory is easier to compensate. However, those are usually very light rounds and subject to wind more than heavier rounds. Cross wind is hard to predict.

The best example I can think of is the 5.56mm NATO and 300 Blackout. The 300 Blackout is a 5.56x45 case necked up to accept a 30 caliber round of either the same weight as the 7.62mm Soviet (123 gr) or a much larger 220 gr round for subsonic and suppressed purposes.

From the 14.5 in (370 mm) of the M4 Carbine, the M855 5.56×45mm round has an effective point target range of 500 meters. The bullet has significant drop, drift, and energy loss at that distance. From a 16 in (410 mm) barrel, a 125 gr (8.1 g) 300 BLK round has a lower velocity and similar bullet drop and drift at shorter distances. However, it has the same amount of energy at 700 meters that the M855 has at 500 meters. In terms of hit probability, the Blackout has an effective range of 460 meters. From a 9 in (230 mm) barrel, the 125 gr BLK round has the same muzzle energy as the M855 from the M4, and 5 percent more energy at 440 meters. In comparison with 7.62×39mm rounds, 300 BLK rounds with varying loads have a better ballistic coefficient and more energy out of similar length barrels. 300 BLK rounds have "barrier blind" performance, being capable of penetration through several inches of different hard targets. The .30 caliber cartridge has an 89.1 percent increase in frontal bullet area over the 5.56×45mm, and so leaves a large wound cavity in soft targets. It also penetrates deeper and initially yaws faster. 300 BLK rounds are effective out of barrels as short as 4.5 in (110 mm).

High muzzle energy is great. But the ability to maintain that energy is also important. If the engagement distance is 50 meters, then I'd go for a 5.7x28mm any day of the week over a full size battle rifle. But if you want accuracy and lethality over long ranges, then small caliber rounds and flechettes don't make sense.

If one could get the electronics small enough, a KETF-style flechette round in .50cal or thereabouts might make more sense in this regard. At full-size KETF in 30 or 35mm reaches out to 3.5 km as a cohesive singular projectile and then - without explosives - shatters into a forward-directed shotgun-style cloud of about 150 bullets of around 35 grain that pepper the target. Should be entirely possible to do that with around 20 "bullets" in a .50cal...